The present invention relates in general to batteries, and, more specifically, to a method and system for determining the relaxation time of a battery cell in an automobile battery in an electric vehicle.
The term “electric vehicle,” includes vehicles having an electric motor for vehicle propulsion, such as battery electric vehicles (BEV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV). A BEV includes an electric motor, wherein the energy source for the motor is a battery that is re-chargeable from an external electric source, such as a power grid. In a BEV, the battery is the source of energy for vehicle propulsion. A HEV includes an internal combustion engine and an electric motor. The energy source for the HEV engine is fuel and the energy source for the motor is a battery. In a HEV, the engine is the main source of energy for vehicle propulsion with the battery providing supplemental energy for vehicle propulsion. The HEV battery buffers fuel energy usage and stores recovered kinetic energy, such as from regenerative braking. A PHEV is like a HEV, but the PHEV has a larger capacity battery that is rechargeable from the external electric grid. In a PHEV, the battery is the main source of energy for vehicle propulsion until the battery depletes to a low energy level, at which time the PHEV operates like a HEV for vehicle propulsion. The electric vehicle monitors the status of the battery using a number of measurements and estimations, including battery state of charge (SOC). An accurate estimate of SOC is needed in the electric vehicle for several purposes, including proper power management and reliable reporting of remaining driving range to the driver.
U.S. Pat. No. 8,706,333, issued Apr. 22, 2014, and U.S. Patent Application Publication 2014/0058595, published Feb. 27, 2014, describe methods and systems for determining battery state of charge and are hereby incorporated by reference.
Lithium ion battery open circuit voltage (OCV) is a good indicator for battery SOC. SOC can be estimated as long as a high quality OCV can be obtained. When using battery terminal voltage as battery OCV, for an accurate SOC estimation, the battery internal diffusion process must complete or nearly complete. If the battery terminal voltage has relaxed for a sufficient time period from the last battery usage, the battery terminal voltage provides a basis for accurate SOC estimation. In contrast, for example, if battery terminal voltage is measured immediately after discharging, SOC can be substantially underestimated. Similarly, SOC can be overestimated when battery terminal voltage is measured immediately after charging.
At power up (i.e., starting a new drive cycle), if the battery voltage has not had sufficient time to relax, a SOC estimate based on the unrelaxed OCV is inaccurate and differs substantially from the most recent SOC estimation obtained during the last drive period. To avoid making an inaccurate SOC estimate, current methods employ fixed waiting times after power off before allowing SOC-OCV adjustment at power up. However, there are drawbacks to using a fixed waiting time. For example, battery voltage may need less time to relax, and if a vehicle is turned on before the preset time elapses from the last drive period, then the battery controller will miss an opportunity to accurately update the SOC value. As another example, preset time periods may be insufficient for voltage relaxation, particularly in conditions at low temperature and heavy battery usage.